corresponding
modifications in the others. Structure and function are bound up
together; every modification of a function entails therefore the
modification of an organ. Hence from the shape of one organ you can
infer the shape of the other organs--if you have sufficiently
extensive empirical knowledge of functions, and of the relation of
structure to function in each kind of organ. Given an alimentary canal
capable of digesting only flesh, and possessing therefore a certain
form, you know that the other functions must be adapted to this
particular function of the alimentary canal. The animal must have keen
sight, fine smell, speed, agility, and strength in paws and jaws.
These particular functions must have correspondingly modified organs,
well-developed eyes and ears, claws and teeth. Further, you know from
experience that such and such definitely modified organs are
invariably found with the carnivorous habit, carnassial teeth, for
example, and reduced clavicles. From a "carnivorous" alimentary canal,
then, you can infer with certainty that the animal possessed
carnassial teeth and the other structural peculiarities of carnivorous
animals, _e.g._, the peculiar coronoid process of the mandible. From
the carnassial tooth you can infer the reduced clavicle, and so on.
"In a word, the form of the tooth implies the form of the condyle;
that of the shoulder blade that of the claws, just as the equation of
a curve implies all its properties."[48]

Similarly the great respiratory power of birds is correlated with
their great muscular strength, and renders necessary great digestive
powers. Hence the correlated structure of lungs, muscles and their
attachments, and alimentary canal, in birds.

Not only do systems of organs, by being adjusted to special
modifications of function, influence one another, but so also do parts
of the same organ. This is noticeably the case with the skeleton,
where hardly a facet can vary without the others varying
proportionately, so that from one bone you can up to a certain point
deduce all the rest.

We deduce the necessity, the constancy, of these co-existences of
organs from the observed reciprocal influence of their functions. That
being established, we can argue from observed constancy of relation
between two organs an action of one upon the other, and so be led to a
discovery of their functions. But even if we do not discover the
functional interdependencies of the parts, we can use th